The bucket shift approach assumes parallel changes in the forward rates implicit in the region of the curve being investigated.

From: FRM NOTES3, <KEY RATE AND BUCKET EXPOSURES>, P61.

"Bucket shifts are similar to key rate shifts but differ in two respects. First, bucket analysis ususally uses very many buckets while key rate analysis tends to use a relatively small number of key rates. Second, each bucket shift is a parallel shift of forward rates as opposed to the shapes of the key rate shifts...."excerpts; fixed income securites-tools for today's market